In an average person, the motion of the knee between the femur and the tibia is not unique. It varies with the person's muscle activity and the functions being performed.
Numerous studies have been performed on both the living knee and cadaveric specimens which determined general characteristics of knee motion, including a neutral path, and deviations about the neutral path which occur when shear forces or torques are superimposed.
Reference data for the normal knee has been obtained using fluoroscopy (Dennis, Komistek et al, 2001), as well as in a variety of other ways on both the living knee and in cadavers. It is now known that during flexion, the medial femoral condyle remains at an almost constant position on the tibial surface, whereas the lateral femoral condyle is displaced posteriorly, off the very back of the tibia in extreme flexion (Iwaki, Pinskerova et al, 2000; Nakagawa, Kadoya et al, 2000).
This movement pattern has been described as a synchronous flexion of the femur about an epicondylar axis and an internal tibial rotation about a vertical axis passing through the medial side of the tibia (Hollister, Jatana et al, 1993; Churchill, Incavo et al, 1998). Variations in the magnitude of the lateral displacement and the tibial rotation have occurred depending on the initial position of the feet on the ground and the activity performed, accommodated by the laxity of the knee (Hill, Vedi et al, 2000).
A relatively stable medial side has been a common factor in the above studies, except for a few millimeters of rollback and even upward levering in extreme flexion due to entrapment of the medial meniscus and impingement of the thigh on the calf. (Li et al, 2003; Conditt et al, 2006; Dawson et al, 2005; Yao et al, 2006; Most et al, 2005).
This normal motion has usually been disrupted however after Total Knee Replacement (TKR), as determined from fluoroscopy studies (Dennis, Komistek et al, 2003). In a deep knee bend, as the knee has flexed, there has been an anterior, rather than posterior, displacement of the femur on the tibia termed ‘paradoxical motion’. The magnitude of internal rotation has been much less than normal on average. The effective pivot location has been variable, ranging from the medial side, the center, and the lateral side.
A striking finding has been the highly variable results from patient to patient. These findings are likely to be due to variations in the preoperative condition of the knees including muscles and soft tissues, to the resection of one or both cruciate ligaments, to the surgical placement of the components, and to the design of the TKR itself. In studies using other techniques, during various flexion-extension activities, the angle of the patella ligament to the long axis of the tibia was found to change from positive to negative during flexion in normal knees but remained almost constant after PCL retaining or substituting TKR (Pandit, Ward et al, 2005).
In studies where the neutral paths of motion were compared in specimens before and after TKR using a robot tester, a reduction of internal tibial rotation and posterior displacement after TKR compared with normal were common findings (Most, Li et al, 2005).
An additional factor is that the A-P stability of the medial side of the normal knee has not been present in a total knee (Blaha 2004). The kinematic abnormalities may reduce the maximum flexion angle achieved, reduce the efficiency of the quadriceps, alter patella mechanics, and not give the ‘feeling of a normal knee’ (Pritchett 2004).
While total knee replacement has been clinically successful, further functional improvements could possibly be made if the kinematics after a TKR more closely matched the intact state. Hence one possible design criterion relating to kinematics is that ‘the neutral path of motion, and the laxity characteristics about that neutral path, is the same for an intact knee specimen, and after implantation of the total knee.’
In theory, this would result in knee kinematics in the living knee with the total knee implanted, the same as that of the knee in its normal intact state. In this context, laxity is defined by the shear force versus displacement, and torque versus rotation curves at a full range of flexion angles. This criterion has the limitation that an off-the-shelf total knee needs to be based on ‘average’ geometry and kinematics, and hence there may not be an exact match for any particular knee.